This invention relates to antennas, phased array antennas, and specifically to a method and structure for interconnecting elements of a phased array antenna.
Phased array antennas offer significant system level performance enhancement for advanced communications, data link, radar, and satellite communications (SATCOM) systems. The ability to rapidly scan the radiation pattern of the array allows the realization of multi-mode operation, LPI/LPD (low probability of intercept and detection), and A/J (antijam) capabilities. One of the major challenges in phased array design is to provide a cost effective and environmentally robust interconnect scheme for the large number of phase shifters within the phased array assembly.
It is well known within the art that the operation of a phased array is approximated to the first order as the product of the array factor and the radiation element pattern as shown in Equation 1 for a linear array 10 of FIG. 1 where EA(θ) is the radiation pattern of the array as a function of scan angle θ.                     ⁢          Equation      ⁢                          ⁢      1                          E        A            ⁡              (        θ        )              ≡                                        E            p                    ⁡                      (                          θ              ,              ϕ                        )                                    ︸                      Radiation            ⁢                                                  ⁢            Element            ⁢                                                  ⁢            Pattern                              ⁢                                    [                                          exp                ⁡                                  (                                                            -                      j                                        ⁢                                                                  2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  r                          o                                                                    λ                                                        )                                                            r                o                                      ]                                ︸                          Isotropic              ⁢                                                          ⁢              Element              ⁢                                                          ⁢              Pattern                                      ·                                            ∑              N                        ⁢                                          A                n                            ⁢                              exp                ⁡                                  [                                                            -                      j                                        ⁢                                                                  2                        ⁢                        π                                            λ                                        ⁢                    n                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                                          x                      ⁡                                              (                                                                              sin                            ⁢                                                                                                                  ⁢                            θ                                                    -                                                      sin                            ⁢                                                                                                                  ⁢                                                          θ                              o                                                                                                      )                                                                              ]                                                                          ︸                          Array              ⁢                                                          ⁢              Factor                                          
Standard spherical coordinates are used in Equation 1 and θ is the scan angle referenced to bore sight of the array 10 in FIG. 1. Introducing phase shift at all radiating elements 15 within the array 10 in FIG. 1 changes the argument of the array factor exponential term in Equation 1, which in turns steers the main beam from its nominal position. Phase shifters are RF devices or circuits that provide the required variation in electrical phase. Array element spacing, Δx or Δy of FIG. 1, is related to the operating wavelength and it sets the scan performance of the array 10. All radiating element patterns are assumed to be identical for the ideal case where mutual coupling between elements does not exist. The array factor describes the performance of an array 10 of isotropic radiators 15 arranged in a prescribed grid as shown in FIG. 1 for a two-dimensional rectangular array grid 10.
To prevent beam squinting as a function of frequency, broadband phased arrays utilize true time delay (TTD) devices rather than traditional phase shifters to steer the antenna beam. Expressions similar to Equation 1 for the TTD beam steering case are readily available in the literature.
The isotropic radiation element 15 in FIG. 1 has infinitesimal dimensions, as explained in subsequent paragraphs. The spacing of the isotropic radiators 15 determines the scan performance of the phased array 10. The elements 15 must be spaced less than or equal to one half wavelength (λo/2) apart for the radiated pattern to be free from grating lobes. Grating lobes are false undesired beams having strength equal to the main beam. The wider the element spacing, Δx or Δy, the smaller the grating lobe-free scan volume is for the array 10. Array factors are also available for 2-D and 3-D phased arrays having rectangular and hexagonal grid arrangements, but they are not discussed here for the sake of brevity.
The isotropic radiating element 15 is an infinitesimally small, nonphysical mathematical concept that is useful for array analysis purposes. On the other hand, all operational arrays utilize physical radiating elements 25 of finite size as shown in the array 20 of FIG. 2. Radiating element size in the plane of a planar array, or along the array surface for a conformal array, is usually a large fraction of λo/2, as required for efficient radiation. Since the array spacing, Δx or Δy, sets the grating lobe-free scan volume of the array 20, it also puts restrictions on the transverse size of the individual radiating elements 25 within the array 20. The extremities of neighboring radiating elements 25 are frequently very close to one another and in some cases, the array spacing, Δx or Δy, prevents certain types of radiating elements 25 from being used.
A comparison of FIGS. 1 and 2 illustrates how real, physical radiating elements 25 consume the majority of the surface area around the array grid intersection points. The array element spacing, Δx or Δy, and transverse size restrictions are further exacerbated in electronically scanned phased arrays. The most general two-dimensional, or three-dimensional (arbitrarily curved surface) electrically scanned phased array antennas require phase shifters at each radiating element 25 to electronically scan the main beam of the radiation pattern. A very space-efficient interconnect cable assembly is required to provide the proper control signals, bias and chassis ground to each individual radiating element 25 and the phase shifters (not shown). However, the physical size of the cabling assembly is often too large and cumbersome to effectively route around the array radiating elements 25 without perturbing the RF field of the radiating element 25 and/or the aggregate field of the sub-array or top-level array assemblies.
What is needed is an interconnect scheme for phased array antennas that is low profile and high density and allows it to be embedded within the phased array structure.